One manner of recovering valuable magnesium products from natural magnesium silicates involves a treatment and extraction procedure whereby the magnesium product is recovered as a soluble magnesium salt which is separated from an insoluble silica residue. Such a procedure involves an acid treatment of the natural magnesium silicate. As an example there may be mentioned the use of ammonium sulfate in U.S. Pat. No. 4,277,449, sulfuric acid in U.S. Pat. No. 2,402,370, ammonium bisulfate in U.S. Pat. No. 3,338,667, sulfur dioxide in U.S. Pat. No. 1,865,224, hydrochloric acid Can. Pat. No. 1,034,385 and carbonic acid U.S. Pat. No. 4,058,587.
It is believed that a second approach might involve a procedure whereby the magnesium product would be recovered as an insoluble product while the silica would be recovered in the form of a soluble product which is the opposite of what has been done in the prior art.
Silica is an acidic substance, therefore, the leaching agent has to be basic. Using such a basic agent such as sodium oxide or potassium oxide or precursor of such oxide such as sodium or potassium carbonate would, in principle, leave all of the impurities carried by the starting silicate as contaminants in the magnesium oxide, since these impurities, mainly oxides of transition metals will not dissolve in a basic phase. Therefore, at first glance, the extraction of silica to recover undissolved MgO is not very attractive.
On the other hand treatment of silica with a base is a well known industrial process. For example sand (SiO.sub.2) may be melted with sodium oxide or a source of sodium oxide such as salt cake (sodium sulfate) or soda ash (sodium carbonate) to produce a variety of products designated as sodium silicate having a range of composition from Na.sub.2 O.SiO.sub.2 to Na.sub.2 O.4SiO.sub.2. The resulting molten product, upon dissolution in water, provides a silicate of soda solution, also known as water glass, which is widely consumed as an adhesive paper in the manufacture of corrugated-paper boxes and in Portland cement products.